Human immunodeficiency virus (HIV) infection is most commonly diagnosed by detecting the appearance of specific antibodies in blood. However, during the earliest stage of infection, rapid replication of HIV makes the immune system dysfunction and for at least several weeks after infection, the immune system cannot produce HIV specific antibodies (Mellors J W, Rinaldo C R Jr, Gupta P, White R M, Todd J A, Kingsley L A (1996) Prognosis in HIV-1 infection predicted by the quantity of virus in plasma. Science 272, 1167-1170). During this period, the virus replicates rapidly, reaching more than a million viral copies per milliliter of blood.
One approach to detecting HIV infection during the early stages of infection is to detect the presence of the HIV virus or its viral components. This can be done using non-direct ELISA methods to detect viral proteins (Gilbert M, Kirihara J, Mills J (1991) Enzyme-linked immunoassay for human immunodeficiency virus type 1 envelope glycoprotein 120. J. Clin. Microbiol. 29, 142-147; Rychert J, Strick D, Bazner S, Robinson J, Rosenberg E (2010) Detection of HIV gp120 in plasma during early HIV infection is associated with increased proinflammatory and immunoregulatory cytokines AIDS Res. Hum. Retroviruses 26, 1139-1145).
One such viral protein is gp120, which is a glycoprotein exposed on the surface of the HIV envelope (Wyatt R, Kwong P D, Desjardins E, Sweet R W, Robinson J, Hendrickson W A, Sodroski J G (1998) The antigenic structure of the HIV gp120 envelope gycoprotein. Nature 393, 705-711; Wyatt, R., and Sodroski, J. (1998). The HIV-1 envelope glycoproteins: fusogens, antigens, and immunogens. Science 280, 1884-1888). The glycoprotein gp120 is anchored to the viral membrane and plays a vital role in virus entry into cells, by binding to CD4 receptors.
ELISA methods for detecting gp120 protein include using pairs of monoclonal antibodies (“capture” and “developer”), which specifically bind to different epitopes of gp120 and do not demonstrate high level of cross-reactivity (Rychert J, Strick D, Bazner S, Robinson J, Rosenberg E (2010) Detection of HIV gp120 in plasma during early HIV infection is associated with increased proinflammatory and immunoregulatory cytokines AIDS Res. Hum. Retroviruses 26, 1139-1145). The average size of these epitopes is 8-10 amino acids, which does not allow for high affinity or efficient binding with the antigen. The sensitivity of this method, thus, is strongly dependent on the antigen-binding efficiency of pair of antibodies, which can vary significantly.